Hopefully when you say draw its just randomly generated and not directly taken from thier deck because that would be uber powerful and this card would give rise to hundreds of mill decks that would flood the arena. Even if it didn't this card is broke 3 ways to sunday. Mindgate 2 turns later for free and definitly going to fill my hand full of cards, with an empty hand thats an 8 card advantage swing and would most likely win you the game. The reason SoSe is so awesome is because it's a 2 card swing for only 3 random quantum.
Personally I think that any card giving "draw cards = to" is going to be too OP for EtG in it's current state.
1. About the general power of the card. For one, the max cards it can mill per Entropy Animus is 7. Second, this assumes that you have 6+ entropy creatures still in your deck and you can play all the cards in your hand. Playing all the cards in your hand is a problem when using this card repeatedly, as you are drawing what the opponent would have drawn and what the opponent has been prepared to power with the previous half of his deck. Surely you don't expect to be able to match the opponent's quanta generation and play all the entropy creatures that you've drawn from your deck?
2. About its power in relationship to Mindgate. It's not free, and Mindgate can be used more than once. The best part about Mindgate is being able to not only match/surpass your opponent's draw advantage but also to maintain that advantage for the rest of the game. This card's usage is much more limited because of the high cost in deck space (entropy quanta generation and entropy creatures, in addition to the Entropy Animi).
3. About its power in relationship to Serendipity. This card costs almost double in quanta and is not an instant spell, i.e. it can be countered easily. In addition, Serendipity's cards are usually of different elements, making them easier to power by (the superior to mono) rainbrow quanta generation. The cars that are gained from Entropy Animus, however, may very well be of one or two elements. So even if you gain more cards, it'll be disproportionately more difficult to use them.
Regarding
1) Any rainbow can generate enough quantum to power an opponents deck and when this goes off its like a first turn with his deck on an empty hand. 6 lycans and however many maxwells should suffice, double for arena. And a 7 card mill is alot when a good portion of decks are 30 cards.
2) Your right. I did mean that it's "free" the turn the ability goes off.
3) Good point. Though SoSe uses quantum towers much like this would prolly use quantum towers so you'll prolly be able to use them as effectively + you get more of them.
And all this is just this card in a few decks, but in those decks this thing can be sick.
Edit*
An added note, this card gives nightmare another use besides Ghostmare.